Mass flow and energy analysis of control systems is the focus of this lecture Conservation of mass Mass and volume flow rates Mass balance for a steady flow process Mass balance for incompressible flow Flow work and the energy of a flowing fluid

1. KEITH VAUGH
MASS & ENERGY ANALYSIS
OF CONTROL SYSTEMS
Reference text: Chapter 6 - Fundamentals of Thermal-Fluid Sciences, 3rd Edition
Yunus A. Cengel, Robert H. Turner, John M. Cimbala
McGraw-Hill, 2008
Lecture 6
Keith Vaugh BEng (AERO) MEng

2. KEITH VAUGH
Develop the conservation of mass principle.
Apply the conservation of mass principle to various systems
including steady- and unsteady-flow control volumes.
Apply the first law of thermodynamics as the statement of the
conservation of energy principle to control volumes.
Identify the energy carried by a fluid stream crossing a control
surface as the sum of internal energy, flow work, kinetic
energy, and potential energy of the fluid and to relate the
combination of the internal energy and the flow work to the
property enthalpy.
Solve energy balance problems for common steady-flow
devices such as nozzles, compressors, turbines, throttling
valves, mixers, heaters, and heat exchangers.
Apply the energy balance to general unsteady-flow processes
with particular emphasis on the uniform-flow process as the
model for commonly encountered charging and discharging
processes.
OBJECTIVES

3. KEITH VAUGH
CONSERVATION
OF MASS
Conservation of mass
Mass, like energy, is a conserved property, and it cannot be
created or destroyed during a process.
Closed systems
The mass of the system remains constant during a process.
Control volumes
Mass can cross the boundaries, and so we must keep track of
the amount of mass entering and leaving the control volume.

4. KEITH VAUGH
Mass is conserved
even during chemical
reactions
Mass m and energy E can be converted to each other according to
where c is the speed of light in a vacuum, which is c = 2.9979 × 108 m/s.
The mass change due to energy change is absolutely negligible.

5. KEITH VAUGH
MASS & VOLUME
FLOW RATES
Mass flow rate
The average velocity Vavg is defined as the
average speed through a cross section.
Definition of average
velocity
Volumetric flow rate
The volume flow rate is the
volume of fluid flowing through
a cross section per unit time.

6. KEITH VAUGH
CONSERVATION OF
MASS PRINCIPLE
The conservation of mass principle for a control
volume
The net mass transfer to or from a control volume during a
time interval Δt is equal to the net change (increase or
decrease) in the total mass within the control volume during
Δt.

7. KEITH VAUGH
Conservation of mass principle for an ordinary
bathtub
General conservation of mass
General conservation of
mass in rate form

8. KEITH VAUGH
MASS BALANCE
FOR STEADY
FLOW PROCESSES
During a steady-flow process, the total amount of
mass contained within a control volume does not
change with time (mCV = constant).
Then the conservation of mass principle requires that
the total amount of mass entering a control
volume equal the total amount of mass leaving it.

9. KEITH VAUGH
Conservation of mass principle for a two
inlet - one outlet steady flow system
For steady-flow processes, we are
interested in the amount of mass flowing
per unit time, that is, the mass flow rate
Many engineering devices such as nozzles,
diffusers, turbines, compressors, and
pumps involve a single stream (only one
inlet and one outlet).
Multiple inlets and exits
Single stream

10. KEITH VAUGH
Special case: Incompressible Flow
The conservation of mass relations can be simplified even further
when the fluid is incompressible, which is usually the case for
liquids.
There is no such thing as a “conservation of volume” principle.
However, for steady flow of liquids, the volume flow rates, as well
as the mass flow rates, remain constant since liquids are
essentially incompressible substances.
During a steady-flow process,
volume flow rates are not
necessarily conserved
although mass flow rates are.
NOTE - Higher density of air at
exit of compressor

11. KEITH VAUGH
MASS & ENERGY ANALYSIS
OF CONTROL SYSTEMS
Reference text: Chapter 6 - Fundamentals of Thermal-Fluid Sciences, 3rd Edition
Yunus A. Cengel, Robert H. Turner, John M. Cimbala
McGraw-Hill, 2008
Lecture 8 (part 2)
Keith Vaugh BEng (AERO) MEng

12. KEITH VAUGH
FLOW WORK &
THE ENERGY of a
FLOWING FLUID
Flow work, or flow energy
The work (or energy) required to push the mass into or out of the
control volume. This work is necessary for maintaining a continuous
flow through a control volume.
In the absence of acceleration, the
force applied on a fluid by a piston
is equal to the force applied on the
piston by the fluid.

13. KEITH VAUGH
Schematic for flow work

14. KEITH VAUGH
TOTAL ENERGY of
a FLOWING FLUID
But the combination Pv + u has been previously defined as the enthalpy h
The flow energy is automatically taken care
of by enthalpy. In fact, this is the main
reason for defining the property enthalpy.

15. KEITH VAUGH
The total energy consists of three parts for a non-
flowing fluid and four parts for a flowing fluid.

16. KEITH VAUGH
ENERGY
TRANSPORT by
MASS
The product ṁiθi is the energy
transported into control volume by
mass per unit time
Amount of energy transport
Rate of energy transport

17. KEITH VAUGH
When the kinetic and potential energies
of a fluid stream are negligible
The product ṁiθi is the energy
transported into control volume by
mass per unit time.
When the properties of the mass at each
inlet or exit change with time as well as
over the cross section

18. KEITH VAUGH
Many engineering systems such
as power plants operate under
steady conditions.
ENERGY
TRANSPORT BY
MASS

19. KEITH VAUGH
Under steady-flow conditions, the mass
and energy contents of a control volume
remain constant.
Under steady-flow conditions, the fluid
properties at an inlet or exit remain
constant (do not change with time).

20. KEITH VAUGH
Mass and energy balances for a steady-flow process
A water heater in steady operation.
Energy balances
Mass balance
}

21. KEITH VAUGH
Energy balance relations with sign
conventions (i.e., heat input and work output
are positive)
Refer to pages, 212 - 214

22. KEITH VAUGH
STEADY FLOW
ENGINEERING
DEVICES
Many engineering devices operate essentially under the
same conditions for long periods of time. The components
of a steam power plant (turbines, compressors, heat
exchangers, and pumps), for example, operate nonstop for
months before the system is shut down for maintenance.
Therefore, these devices can be conveniently analysed as
steady-flow devices.
A modern land-based gas turbine used for
electric power production. This is a General
Electric LM5000 turbine. It has a length of
6.2 m, it weighs 12.5 tons, and produces
55.2 MW at 3600 rpm with steam injection.

23. KEITH VAUGH
Nozzles and Diffusers
Commonly utilised in jet engines, rockets,
spacecraft, and even garden hoses.
A nozzle is a device that increases the velocity
of a fluid at the expense of pressure.
A diffuser is a device that increases the
pressure of a fluid by slowing it down.
The cross-sectional area of a nozzle decreases in
the flow direction for subsonic flows and increases
for supersonic flows. The reverse is true for
diffusers. Nozzles and diffusers are shaped so
that they cause large changes in fluid
velocities and thus kinetic energies.
Refer to Example 6-4 & 6-5, page 216/7

24. KEITH VAUGH
Turbines and compressors
Turbine drives the electric generator in power
plants. As the fluid passes through the turbine, work
is done against the blades, which are attached to the
shaft. The shaft rotates, and the turbine produces
work.
Compressors, as well as pumps and fans,
increase the pressure of a fluid. Work is supplied to
these devices from an external source through a
rotating shaft. Energy balance for the compressor in
this figure:
Refer to Example 6-6 & 6-7, page 218/9
A fan increases the pressure of a gas slightly and is
mainly used to mobilise a gas.
A compressor is capable of compressing the gas to
very high pressures.
Pumps work very much like compressors except
that they handle liquids instead of gases.

25. KEITH VAUGH
Throttling valves
Throttling valves are any kind of flow-
restricting devices that cause a significant
pressure drop in the fluid.
The pressure drop in the fluid is often
accompanied by a large drop in temperature
and for that reason throttling devices are
commonly used in refrigeration and air-
conditioning applications.
Energy balance
The temperature of an ideal gas does not
change during a throttling (h = constant)
process since h = h(T).
During a throttling process, the enthalpy of a
fluid remains constant. But internal and flow
energies may be converted to each other.
Internal energy + Flow Energy = Constant
Refer to Example 6-8, page 221

26. KEITH VAUGH
Mixing chambers
In engineering applications, the section where the
mixing process takes place is commonly referred
to as a mixing chamber.
Energy balance for the adiabatic
mixing chamber in the figure
The T-elbow of a shower or tap
serves as the mixing chamber for
the hot and the cold-water streams.
10C
60C
43C
140 kPa
Refer to Example 6-9, page 223

27. KEITH VAUGH
Heat exchangers
Heat exchangers are devices where two moving
fluid streams exchange heat without mixing. Heat
exchangers are widely used in various industries
and come in a variety of designs.
A heat exchanger can be as simple
as two concentric pipes
The heat transfer
associated with a heat
exchanger may be
zero or nonzero
depending on how the
control volume is
selected.

28. KEITH VAUGH
Mass and energy balances for
the adiabatic heat exchanger in
the figure are:
Refer to Example 6-10, page 224

29. KEITH VAUGH
Pipe and duct flow
The transport of liquids or gases in pipes and
ducts is of great importance in many engineering
applications. Flow through a pipe or a duct usually
satisfies the steady-flow conditions.
Pipe or duct flow may involve more than
one form of work at the same time.
Heat losses from a hot fluid flowing through
an uninsulated pipe or duct to the cooler
environment may be very significant.

30. KEITH VAUGH
Energy balance for the pipe flow
shown in the figure is
Refer to Example 6-11, page 227

31. KEITH VAUGH
ENERGY ANALYSIS
OF UNSTEADY
FLOW PROCESSES
Many processes of interest, however, involve changes
within the control volume with time. Such processes
are called unsteady-flow, or transient-flow, processes.
Most unsteady-flow processes can be represented
reasonably well by the uniform-flow process.
Uniform-flow process
The fluid flow at any inlet or exit is uniform and
steady, and thus the fluid properties do not change
with time or position over the cross section of an inlet
or exit. If they do, they are averaged and treated as
constants for the entire process.

32. KEITH VAUGH
Charging of a rigid tank from a
supply line is an unsteady-flow
process since it involves changes
within the control volume.
The shape and size of a control
volume may change during an
unsteady-flow process.

33. KEITH VAUGH
Mass balance
Energy balance
{
i = inlet
e = exit
1 = initial state
2 = final state

34. KEITH VAUGH
The energy equation of a uniform-flow
system reduces to that of a closed system
when all the inlets and exits are closed.
A uniform flow system may involve
electrical, shaft and boundary work
all at once

35. KEITH VAUGH
Conservation of mass
Mass and volume flow rates
Mass balance for a steady flow process
Mass balance for incompressible flow
Flow work and the energy of a flowing fluid
Energy transport by mass
Energy analysis of steady flow systems
Steady flow engineering devices
Nozzles and diffusers
Turbines and compressors
Throttling valves
Mixing chambers and heat exchangers
Pipe and duct flow
Energy analysis of unsteady flow processes